A(1). Field of the Invention
The invention relates to a video recorder arranged to receive a composite color video signal and to record it on a magnetic tape or another means. This recorder is further arranged to convert the information recorded on the magnetic tape into the original composite color video signal which can be applied to a standard color TV receiver for display on a TV screen.
A(2). Description of the Prior Art
As is known, a composite color video signal is formed by the sum of a luminance signal y(t) and a chrominance signal chr(t). The luminance signal has a frequency spectrum Y(f) located in the frequency band extending from 0 Hz to approximately 5.5 MHz. The chrominance signal has a frequency spectrum CHR (f) located within the frequency band of the luminance signal, more specifically in that portion of this frequency band located between the frequencies 3 MHz and 5.5 MHz.
A composite color video signal may alternatively be considered as a sequence of line signals which each correspond to a picture line of the TV-picture and are each present during a period of time T.sub.1 which is designated the line period.
The magnetic tape on which the above-mentioned composite color video signal must be recorded, may be considered to be a transmission medium having a transmission band which extends from a low frequency f.sub.A to a higher frequency f.sub.B. Herein f.sub.A is considerably higher than zero Hertz; in practice it is approximately 200 kHz. The frequency f.sub.B is inter alia determined by the tape speed. The higher the tape speed, the higher f.sub.B, but also the greater the quantity of tape "consumed" per second. In addition to the fact that no low frequency component can be transmitted via this transmission medium, it introduces transmission noise, the energy of which is uniformly distributed over the overall transmission band.
If the composite color video signal is directly recorded on the magnetic tape, then the quality of the recovered video signal is very poor because it no longer contain low-frequency luminance components and because the luminance and chrominance signals are both highly affected by the transmission noise.
In order to prevent the loss of low-frequency luminance components, it has already been proposed to modulate the color video signal on a carrier signal which has such a high carrier frequency that the lowest frequency component in the modulated color video signal is well above f.sub.A. In addition, frequency modulation is applied so that the distribution of the energy of the transmission noise over the transmission band is shaped. More specifically, frequency modulation reduces the noise energy for the lower frequencies and inceases the noise energy for the higher frequencies in the transmission band. In order to prevent the chrominance signal, and consequently the color rendering, from being increasingly adversely affected by this shaping of the noise energy, a high carrier frequency of, for example, 10 MHz and a very high tape speed are required. Although a very good transmission quality is accomplished by means of this transmission method, it is only used in professional video recorders. For video recorders for the consumer's market, the very high tape speed is unacceptable.
A different transmission method is therefore used in the last-mentioned video recorders, as a result of which a reasonable transmission quality is accomplished with a low tape speed. More specifically, the chrominance signal is separated from the luminace signal by means of a high or band-pass filter. A carrier signal which has a nominal frequency of approximately 4 MHz is frequency-modulated with the luminance signal thus obtained. Of the resultant FM-signal, that portion is selected which is located in the frequency band extending from approximately 1 MHz to approximately 7 MHz. This portion is directly recorded on the magnetic tape. That portion of the transmission band which is located between the frequencies f.sub.A and 1 MHz is now used to transmit the chrominance signal. To that end, the bandwidth of the separated chrominance signal is first very greatly limited to approximately 4 kHz and is thereafter converted by means of amplitude modulation to the frequency band extending from f.sub.A to approximately 1 MHz and this converted signal is directly recorded on the magnetic tape.
The transmission method, which is known as "color under", has the disadvantage that the bandwidth of the luminance signal is very greatly limited, as a result of which much luminance information is lost. As this luminance signal is transmitted over the transmission channel by means of frequency modulation, the influence of the transmission noise on this luminance signal is acceptable.
This transmission method has the further disadvantage that the chrominance signal is transmitted over the transmission channel by means of amplitude modulation, the carrier frequency in practice being 562 kHz. As a result thereof, no shaping is obtained of the distribution of the noise energy of the transmission noise present in that portion of the transmission band which extends from f.sub.A to approximately 1 MHz. This results in this converted chrominance signal being significantly affected by the transmission noise. As moreover the bandwidth of the converted chrominance signal is considerably smaller than the bandwidth of the original chrominance signal, the transmission quality of the chrominance signal is not very good in this transmission method.